1. Field of the Invention
The present invention relates generally to magnetic recording and playback apparatus, and more particularly relates to such apparatus in which a level indicator is selectively connected to any of various amplification stages to indicate a corresponding signal level.
2. Descriiption of the Prior Art
There are many conventional stereo or high fidelity audio systems in which a tape recorder and an amplifier are combined as a unit with other components, such as a record player, an AM/FM tuner, or the like.
Such a stereo or other high fidelity system generally includes a source selector disposed on the front panel of the unit for selectively connecting one of the tape recorder, record player, and tuner to the amplifier to provide thereto an audio signal. Normally, there is also included on the front panel a VU meter or other signal level indicator, which can be, for example, a moving-needle type meter or a bar-graph LED display.
The VU meter is normally connected to measure the output level of the power amplifier used to drive speakers or head phones. However, when a tape recording mode is selected, the VU meter is switched to a line amplifier connected with the recording head of the tape recorder, so that the VU meter displays the signal level of the signal being recorded. In a tape playback mode, the VU meter is connected to the power amplifier, rather than to the line amplifier. Therefore, in the playback mode, it is not possible to ascertain, by viewing the VU meter, whether the signal being picked up from magnetic tape is being provided at an appropriate level. Thus, it is not possible to determine whether the line amplifier requires adjustment to achieve an optimum signal, or whether the signal has been previously magnetically recorded at an optimum level.
In order to overcome the above difficulty, several configurations have been previously proposed.
According to one previously-proposed configuration, a manually operated change-over switch is provided on the front panel of the unit for selecting one or the other of the power amplifier and the line amplifier to be coupled to the VU meter. Such an arrangement does have the advantage of making it possible to indicate the recording signal level when the recording mode is selected, and also to indicate the level of recorded signal on the magnetic tape when the playback mode of the tape recorder is selected. However, the manual operation required for the change-over switch tends to induce operator errors. More particularly, an operator may believe that the output of the power amplifier is being indicated on the VU meter, when, instead, the output of the line amplifier is actually being indicated. As a result, the operator may unintentionally misadjust the setting of either the line amplifier or the power amplifier, because of his misperception of what the VU meter indicates.
Another proposed arrangement to avoid the above-mentioned difficulties involves arranging the change-over switch to be operated in conjunction with a source selector switch coupled to the power amplifier and having a source contact and a tape play contact. When the source selector switch is actuated to its source contact, a source signal, such as the output of the record player or tuner, is furnished to the power amplifier, and the output signal level of the power amplifier is furnished to the VU meter to show the volume of the output signal. When the select switch is actuated to its tape play contact, the playback signal from the tape recorder is furnished through the select switch to the power amplifier, while the output signal level of the line amplifier is furnished to the VU meter. While this arrangement tends to eliminate the problem of operator error, it is impossible with this arrangement for the VU meter to indicate the volume of the output signal from the power amplifier when a tape playback mode has been selected.
It should be remarked that in the conventional or prior art stereo or high fidelity system, the VU meter does not show a significant response when the sound volume is extremely low. For example, if the VU meter is a moving-needle meter, changes in level will not produce noticeable swing of the meter needle. Also, when a bar-graph meter, or other linear arrangement of light emitting diodes is used, the light emitting states of the light emitting diodes will not vary enough to indicate changes in level when the volume is extremely low, so that the optical effect of the bar-graph representation is considerably degraded. This drawback makes it difficult for a retail shop to provide a satisfactory display of equipment performance of a tape machine when in its playback mode, if the sound volume is turned down to a low level.